@TheBobkin yeah, that's a hard one for folks to grasp sometimes. The Witness node in a stretched or 2-node simple is a dedicated node for metadata to perform tie-breaking, but in other vSAN cluster this isn't needed and the Witness component exists on any node as well as data.
As for the placement, it does seem random for sure, but I have seen that if I take one of my 3 nodes out for an extended period of time or I did a full data migration when entering Maint Mode, there is a rebalance that occurs afterwards. I've always assumed that this is due to the vSAN cluster setting of Automatic rebalance, which is not enabled by default, to ensure that all disks are within the threshold. Otherwise, isn't there a Skyline alert that get's generated when things are too far out of balance?
Both good points for the OP to know when going into a non-stretched cluster, so good talking points. I will admit, I am still new at vSAN, so there's a lot of in-depth details I am still learning, but answering these posts and looking at other answers definitely helps to expand my knowledge base.
